1. Field of the Invention
The present invention relates to a method for the production of graphite bodies, as well as the graphite bodies produced therewith.
2. Description of the Related Art
Graphite bodies are for example graphite electrodes and connecting pieces (nipples) for graphite electrodes. The following explanations relate in particular to graphite electrodes and nipples, but also apply correspondingly to graphite bodies other than graphite electrodes or nipples.
Graphite electrodes are used for steel production in an electric arc furnace. Graphite electrodes must withstand diverse mechanical stresses. In addition to temperature induced thermal material stresses, these include the mechanical stresses which arise on insulating elements during tilting of the furnace, due to vibration, due to dislocation of scrap during melting, as well as during placement of the strand into the scrap. In order to withstand the conditions in an electric arc furnace it is therefore essential that the relevant graphite electrode has a low coefficient of thermal expansion, low electrical resistance, sufficient density and the properties associated therewith (strength and E-modulus).
Needle cokes, such as petroleum-based or pitch-based needle cokes are generally used in the production of graphite electrodes. The quality and value of needle coke which has a needle-like crystalline structure is determined by a number of factors such as, for example sulfur and nitrogen content, hardness, ash content, electrical resistance and coefficient of thermal expansion. Although each of these factors is important, the primary measure of quality of needle coke is the linear coefficient of thermal expansion or CTE. In order to be considered premium coke, in other words very high-quality coke, a CTE of less than 0.5 [μm/mK] is generally assumed (measured according to DIN 51 909 of Oct. 1989 in a temperature range of 20° C. to 200° C.). The lower the CTE of the needle coke, the less will be the linear expansion of the graphite electrode produced with this needle coke when used in the electric arc furnace and the greater will be the corresponding thermal shock resistance.
In order to produce graphite bodies, such as graphite electrodes and nipples, two graphitization methods are generally used.
One method—the Acheson-graphitization method—was initially described in US patent document 702,758. The furnace consists of a horizontal bed of refractory materials, furnace heads that carry the current load to the furnace and long side walls, consisting of concrete blocks, steel plates or steel grids. The bottom is cooled by air and loaded with a layer of insulating material, for example granular silicon carbide, metallurgical coke, sand or sawdust. The same materials are used to insulate the side walls and the surface of the furnace.
Due to cheaper production costs however, the second method—the Castner- or lengthwise graphitization method—is the currently commonly used method in the production of graphite electrodes. The material to be graphitized is heated as an ohmic resistance in the direct passage of current to above 2200° C. to 3000° C. The original device for implementation of lengthwise graphitization is the subject of U.S. Pat. No. 1,029,121.
Nipples are used to connect graphite electrodes with one another. The graphite electrodes are herein equipped on their face with threaded boxes into which the nipples are screwed. Production of these nipples occurs generally also on the basis of needle cokes.
Because of the irreversible thermal volume expansion of the needle cokes, the process curve for graphitizing of, for example electrodes or nipples must be adapted very precisely to the parameters of the needle cokes, which results in higher production costs. However, this is the only way to date to reduce the invisible, under the surface defect rate that leads to breakdowns in subsequent use in steel mills.
On the one hand, high quality premium cokes are expensive since they are available only on a limited basis. On the other hand—as previously mentioned—a very defined process curve must generally be applied during graphitizing of premium cokes, resulting in higher production costs.
What is needed in the art is a method for the production of graphite bodies, replacing high quality needle cokes with a more cost-effective base material with which graphite bodies can be produced that have properties that are comparable with high quality needle cokes.